1. Field of the Invention
The present invention relates to power amplifiers and more particularly to a wideband driver for a Class-D power amplifier.
2. Description of the Related Art
Implementation of high efficiency class-D amplifiers has been limited to HF frequencies due to the difficulty of operating the power transistors as ideal switches at higher frequencies such as VHF/UHF range and beyond.
FIG. 1 (Prior Art) shows a classical driving circuit for the complementary voltage-switching class D amplifier. It is difficult to realize the transformer T1 at high frequency since it utilizes a flux linkage method which works from a few KHz up to only about 200 MHz.
U.S. Pat. No. 6,949,978 issued to R. Tayrani et al, entitled “Efficient Broadband Switching-Mode Amplifier,” discloses an amplifier that includes a mechanism for amplifying an input signal via a high-speed switch and providing an amplified signal in response thereto. Another mechanism filters the amplified signal via common mode rejection and provides an output signal in response thereto. In a specific embodiment, the mechanism for filtering includes a first mechanism for separating an input signal into plural intermediate signals. The mechanism for amplifying includes a second mechanism for amplifying the plural intermediate signals via one or more high-speed switches and providing plural amplified signals in response thereto. The mechanism for filtering further includes a third mechanism for employing common mode rejection to filter the plural amplified signals, yielding a single output signal in response thereto. As shown in FIG. 1 of the Tayrani disclosure utilizes one Balun 1:1 transformer at the input. The Balun 1:1 provides two inputs, which are 180 degrees out of phase to drive each following transistor.
U.S. Pat. No. 5,023,566 issued to El-Hamamsy et al, entitled “Driver for a High Efficiency, High Frequency Class-D Power Amplifier,” discloses a driver circuit for a voltage-switching, high frequency Class-D power amplifier provides an input sinewave power signal and controls the transition time between switching the two active devices thereof, resulting in substantially lossless switching. The transition time is optimized by controlling the amplitude of the voltage signals at the inputs of the two active devices, depending on the output capacitance of the switching devices, the threshold voltage of the switching devices, the power output requirement, and the impedance of the resonant load network of the Class-D power amplifier. As shown in FIG. 2 of the El-Hamamsy disclosure one conventional transformer (T1) is used at the input, which is 180 degrees out of phase to drive the following transistors. El-Hamamsy et al use a Class-E amplifier as the driver to provide signals to the conventional transformer. The focus of the disclosure is on controlling the amplitudes of the waveforms into the Class-D amplifiers.
It is desired that power transistors be operated as ideal switches at frequencies higher than the HF frequencies to allow implementation of Class-D amplifiers.